Cell for fused-salt electrolysis with gas collecting means

ABSTRACT

A cell for fused-salt electrolysis for reducing alumina in the presence of cryolite to metallic aluminum. The cell includes an insulated tank having a carbon bottom, two rows of anode rods disposed on opposite sides of an anode carrier positioned in the longitudinal center plane, an exhaust gas-collecting duct extending at the longitudinal center between the two rows of anode and covers downwardly inclined from the outside of respective rows of anode rods to respective edges of the tank and provided at the outer longitudinal edge and the transverse edges with a skirt that extends to the respective edge of the tank. Each cover is hinged adjacent to its longitudinal edge adjacent to the anodes and at least at its corner portions to a support which extends along the row of anode rods. A carrying arm is secured to each end portion of the support and is pivoted at its other end to the anode carrier. The carrying arm is connected by an operating linkage to a piston rod of a cylinder of a hydraulic or pneumatic actuator. A second cylinder of a second hydraulic or pneumatic actuator is pivoted to the support at least adjacent to the middle thereof and the piston rod of the second actuator engages the cover.

This application is a continuation-in-part-application of U.S.application Ser. No. 838,431, filed Sept. 30, 1977, which is acontinuation of U.S. application Ser. No. 645,115, filed Dec. 29, 1975both now abandoned.

BACKGROUND

This invention relates to a cell for fused-salt electrolysis forreducing alumina in the presence of cryolite to metallic aluminum,comprising an insulated tank, which has a carbon bottom, two rows ofanode rods, which are disposed on opposite sides of an anode carrier,which is arranged in the longitudinal center plane, an exhaustgas-collecting duct extending at the longitudinal center between the tworows of anode rods, and two covers, which are downwardly inclined fromthe outside of respective rows of anode rods to respective longitudinaledges of the tank and which are provided each at its outer longitudinaledge and at each of its transverse edges with a skirt that extends asfar as to the respective edge of the tank.

The exhaust gases formed in the electrochemical process of recoveringaluminum from alumina contain fluorine, which is a product of thereaction of the cryolite in the electrolytic cell, and finely dividedcarbon, which released by the carbon anodes. For ecological reasons, itis essential to reduce particularly the fluorine content in the exhaustgases to the utmost minimum before such gases are discharged into theatmosphere so that the environment is not adversely affected.

It is known for this purpose to purify the exhaust air which escapesfrom the buildings which contain plants for fused-salt electrolysis --this air contains the exhaust gases -- and then to discharge the air inan ecologically innocuous state. To this end the natural chimmey actionin the buildings is utilized to move the air in the buildings, whichcontains the exhaust gases from the electrolysis, to mechanical means,such as blowers, purifiers or the like, which are mounted on the roofsand serve to discharge the emissions. The purifiers consist essentiallyof liquid-wetted filters, between which a mist is maintained so thatdust particles and gaseous constituents are retained by the filters andby the liquid particles and the purified air is discharged into theatmosphere.

Whereas the mechanical means which are available at present can handleand purify all air in the buildings, they do not decisively improve theconditions within the building from the aspect of work hygiene.

To maintain the concentration of the noxious exhaust gases which escapefrom the cells for fused-salt electrolysis within tolerable limits, itis known to provide the cells for fused-salt electrolysis with a cover.This cover consists of a collecting duct, which extends longitudinallybetween the rows of anode rods and which is provided at its outerperiphery with two cover plates, which are downwardly inclined towardthe longitudinal side of the tank. The anode rods extend through thecover plates and those edge portions of the cover plates which are nearthe longitudinal sides of the tank closely embrace the anode blocks sothat an uncovered surface of the electrolytic cell remains exposedbetween the inside periphery of the tank and the outside edge of thecover plates and permits of an access to the interior of the cellwhereas the exhaust gases from the cell can be sucked off only from arelatively small part thereof.

It is known to eliminate this disadvantage by the use of a cell forfused-salt electrolysis in which the cover plates are mounted on a tubewhich is longitudinally bisected and which engages the upper portion ofthe periphery of the cylindrical exhaust gas-collecting duct. Hingedplates are mounted at the outside edges of the cover plates and rest onthe longitudinal edges of the tank. The rate at which exhaust gasesescape into the air in the building is much reduced by this arrangement.

On the other hand, the arrangement has the disadvantage that these meansmust be entirely removed for a replacement of the anode.

For this reason it has been proposed (Opened German Specification No.2,251,898) so to design the cover that the same must be opened only whenit is desired to charge the cell and to remove the molten aluminum. Thisobject is accomplished in that the cover covers the anodes on theirvertically extending sides, transverse walls intended to rest on theends of the tank are mounted on the cover, and the anodes consist ofcontinuous anodes. This arrangement is not highly suitable whereso-called manipulators are to be used, which can, e.g., automaticallybreak up the slag crust, and feed the alumina to the cell. Besides, thearrangement requires the use of continuous anodes.

SUMMARY

The invention provides a cell for fused-salt electrolysis with coverswhich are so designed that the electrolytic cell can be operated in asimple manner by a manipulator, the molten material can be removedwithout obstruction, the anodes can be replaced without specialdifficulties, and particularly tight covers are also provided.

This object is accomplished by constructing the cover, which on thelongitudinal sides of the lowerable anode carrier extends inclined tothe cathode tank, in two parts, the lower, pivoting part of the coverbeing hinged in the vicinity of its longitudinal edge adjacent to theanodes, or at least in its corner portions, to a support which extendalong the row of anode rods and which is rigidly connected at least byits end portions to carrying arms pivoted to bearing brackets mounted onthe lowerable anode carrier, the second part of the cover, disposedbetween the support and anode carrier, being constructed essentially toabut on the lowerable anode carrier.

Because the entire anode carrier descends slowly as the anodes areconsumed, the fixation of the bearing bracket to the anode carrierensures that there will be no relative movement between the anodecarrier and the cover so that the structure is very tight.

At least adjacent to the middle of the support, the cylinder of apneumatic actuator is pivoted to an abutment mounted on the support andthe piston rod of said actuator is movably connected to the cover. Thisactuator is operable to impart a sufficient upward pivotal movement tothe cover when it is desired to charge the cell for fused-saltelectrolysis and to remove the molten aluminum.

The lower cover is appropriately secured to the upper cover by means ofa plurality of brackets. The piston rod of the cylinder, which ismounted on the middle carrying arm and actuates the movement of thelower cover, may then be secured to one of the brackets.

Fastening means for a hoisting device may be attached to the two middlecarrying arms in the center portion of the upper cover.

In accordance with an advantageous mode of construction of the apparatusin accordance with the invention, the two-part control linkage which isconnected to the piston rod of a pneumatic of hydraulic cylinder mountedon the anode carrier engages at least one carrying arm. The part of thelinkage which is connected to the piston of the linkage then ispivotably connected to the bearing bracket.

In accordance with another, likewise advantageous mode of construction,fastening means for a hoisting device are provided at least on themiddle cantilevers.

Both arrangements permit the raised cover to be raised further when theanodes are to be replaced, thus making the anodes readily accessible.

According to a special feature of the invention, U-shaped anode rodseals are disposed on that side of the support which faces the exhaustgas-collection duct, which extends as far as to the anode rods, andthese seals embrace the anode rods and positively engage the same sothat the seals can adjust themselves to the instantaneous position ofthe anode rods, which is not exactly fixed.

Each anode rod seal consists of a sliding element, which positivelyembrances the anode rod and which is horizontally slidably mounted in aguide which is mounted on a baseplate.

DESCRIPTION OF THE DRAWINGS

The apparatus in accordance with the invention is shown in the drawingby way of example and is explained in detail below.

FIG. 1 is a front elevation of the cover in the closed position;

FIG. 2 is a side elevation of the cover in the closed position, with acylinder mounted on the anode carrier and with a control linkage

FIG. 3 is a side elevation of the cover in the closed position, withfastening means for a hoisting device;

FIG. 4 shows a slidable anode-rod seal; and

FIG. 5 is a side elevation of the anode-rod seal.

DESCRIPTION

The anode carrier 3 carrying the anode rods 2 is disposed over the cell1 for fused-salt electrolysis and extends in the longitudinal centerplane of the cell, exhaust gas collection ducts 4 being provided betweenthe two rows of anodes. A sheet or plate-like two-part cover 5 and 5aextends downwardly inclined from each row of anodes to the longitudinaledge of the electrolytic cell 1. Skirts 6 of asbestos are mounted on thelongitudinal edge and the later edges of the lower cover 5a and rest onthe longitudinal edge and the lateral edges of the electrolytic cell 1and prevent the escape of exhaust gases in this area. Carrying arms 9are secured to the support tube 8 both at its end portions and at itscenter, the other end of each carrying arm being pivoted to a bearingbracket 10 mounted on the anode carrier 3.

According to FIG. 2, a two-link control linkage 11 connected to thepiston rod of a pneumatic cylinder 12 mounted on the anode carrier 3engages the carrying arm 8. The part of the linkage which is articulatedto the cylinder 12 is pivoted to the bearing bracket 10.

Disposed at the center of the support tube 8 is an abutment 14 to whichthe end of a hydraulic or pneumatic cylinder 13 is pivoted, the pistonrod of said cylinder being pivoted to a bracket 7 mounted on the cover5.

In a mode of construction according to FIG. 3, fastening means for ahoisting device are provided on at least one cantilever 18 in the centerprotion of the upper cover. The lower cover 5a is pivotably connected bymeans of brackets 7 to said cantilver or cantilevers 18, disposed in thevicinity of the longitudinal edge of the lower cover 5a which isadjacent to the anodes.

Disposed on the middle carrying arm 9 is a bearing 19 to which the endof the hydraulic or penumatic cylinder 13 is pivotably connected, thepiston rod of the cylinder being pivoted to the bracket 7.

Pivotal movement I is imparted to the cover by the cylinder 13, andpivotal movement II by the shop crane or by the control linkage 11.

As shown in FIG. 2, the pivot axis at the upper end a of element 11 isfixed on bearing bracket 10. The ends b and c are connected to the otherlinkages at movable pivot points. Pivot point d is fixed on arm 9.

As can be seen from FIGS. 1 and 2, the whole structure is carried by theanode carrier 3. The anode carrier descends with the velocitycorresponding to the anode consumption and thus the whole structurevertically descends with a velocity corresponding to this anodeconsumption. Thusly, the asbestos skirts 6 prevent the cover 5 fromlosing its tight fit because, as shown, the skirts are flexibly thin.From this it is clear that there is no relative movement between thestructure or parts of the structure and the anode carrier during thedescension.

As can be further seen from FIGS. 1 and 2, the abutment 14 is fixed ontothe support arm 8 and moves in the same way as support arm 8, whichmeans there is no relative movement between abutment 14 and support arm8. When cylinder 13, which is moveably fixed onto abutment 14, moves,only the cover 5 is moved in art I.

The arrangement according to the invention permits of a control of eachcover in such a manner that the pneumatic actuator comprising thecylinder 13 is operated to impart an upward pivotal movement to thecover 5 when it is desired to perform general operations in the cell,such as breaking crusts, charging alumina, ladling molten metal andchecking the cells. For a replacement of anodes, a further upwardpivotal movement, beyond the vertical position, is imparted to thepreviously raised cover 5 about the pivotal axis of the carrying arm 9.

FIGS. 4 and 5 show the seals between the anode rods and the cover. Theanode rod 2 is positively embranced by a horizontally slidable element15, which is slidably mounted in a guide 16 on a baseplate 17. Thebaseplate is secured to the supporting tube 8 and directly engages theanode carrier 3.

It will be appreciated that the instant specification and example areset forth by way of illustration and not limitation, and that variousmodifications and changes may be made without departing from the spiritand scope of the present invention.

What is claimed is:
 1. A cell for fused-salt electrolysis for reducingalumina in the presence of cryolite to metallic aluminum, comprising aninsulated tank having a carbon bottom, an anode carrier arranged in thelongitudinal center plane and movable downwardly during use, two rows ofanode rods disposed on opposite sides of the anode carrier, anexhaust-gas collecting duct extending in the longitudinal center betweenthe two rows of anode rods, and a plate-like cover inclined downwardlyfrom the outside of each row of anode rods to the longitudinal edges ofthe tank and provided both at its longitudinal edge adjacent to the tankand at its transverse edges with a skirt extending as far as the edge ofthe tank, a support extending along the rows of anode rods, bearingbrackets mounted on the anode carrier and carrying arms pivotallymounted on the bearing brackets, wherein the cover extends at aninclination to the cathode tank on the longitudinal sides of the anodecarrier and comprises two parts, a lower pivotable part, hinged on atleast a portion of its longitudinal edge portion adjacent to the anoderods to the support which is rigidly connected at least in its endportions to the carrying arms and a second cover part disposed betweenthe support and the anode carrier and abutting the anode carrier.
 2. Acell for fused-salt electrolysis according to claim 1, wherein thesupport is tubular and the lower part of the cover is hinged by bracketsto the tubular support.
 3. A cell for fused-salt electrolysis accordingto claim 1, further comprising cantilevers connected to the lower partof the cover to pivotably move same.
 4. A cell for fused-saltelectrolysis according to claim 1 further comprising a cylinder with anabutment mounted on the support, and a piston rod for the cylinderpivotably connected to a bracket for pivotally moving the lower part ofthe cover.
 5. A cell for fused-salt electrolysis according to claim 2,further comprising a bearing disposed on the middlemost carrying arm, acyclinder pivotally secured to the bearing the piston rod of thecylinder being likewise pivotably connected to the brackets.
 6. A cellfor fused-salt electrolysis according to claim 1 further comprising acylinder mounted on the anode carrier, a two-part control linkageconnected to the piston rod of the cylinder and engaging at least onecarrying arm and a bearing bracket pivotally connected to the part ofsaid linkage which is connected to the piston of the cylinder.
 7. A cellfor fused-salt electroylsis according to one of claim 3, furthercomprising fastening means for a hoisting device are provided on atleast one of the intermediate cantilevers.
 8. A cell for fused-saltelectrolysis according to claim 1, further comprising U-shaped anodeseals which positively embrace the anodes and are secured to the supporton the side thereof adjacent to the anodes.
 9. A cell for fused-saltelectrolysis according to claim 8, wherein the anode seal comprises abaseplate connected to the support, a guide mounted on the baseplate anda slidable member which is horizontally slidable in the guide.